Re: Metering question

[Clive Warren <Clive.Warren@xxxxxxxxxxxxxx>]

At 9:03 pm -0400 12/7/00, Michael Kennedy wrote:

>I haven't shot many rolls of Konica 750--but the lab I worked in process
>about ten for a photographer and they all came out blank. My understanding
>of some filters are that they pretty much cut out all light below 800nm.
>Konica has peak sensitivity at 750(hence the name). Shooting with opaque
>filters is just about the same as shooting with your metal lens cap on.

There are quite a few of us Konica users on the list who shoot 750IR with
89B and 88A filters.  Whilst these filters are not completely opaque, there
is not much visible light allowed through.

If you have a quick look at the film sensitivity curves in the Infrared FAQ
(URL in sig. file) (Section 12 "Which Infrared Filters"), it is clear that
Konica 75IR is sensitive to light from 640 to 820 nm in addition to the
intrinsic  sensitivity range of AgBr 400 to 500nm. The peak sensitivity is
of course 750nm in the infrared range.

Generally, filters also have a sensitivity curve rather than a "cut-off".
So, again looking at the sensitivity curves this time for filters to
Wratten specifications, even an 87C will allow some infrared exposure on
Konica 750.

The exposures required for the more opaque filters will be a lot longer -
see Section 13 "Exposing - Film Speeds and Rating for Filters".  I have not
listed exposure times for Konica 750 for filters above the 89B as the film
would have to be rated at ridiculously slow speeds.  However I have shot
Konica 750 using 88A filters as an experiment.

Essentially when using increasingly opaque filters, film speed decreases
and the contrast is increased in an already high contrast film making it a
bit of a challenge to print.

If the photographer you refer to was using say an 87 or 87C filter and
rated his film higher than 1ASA then blank film (ie underexposed) would be
the result.

It's all in the FAQ :-)

There are some peops on the list who do shoot Konica 750 using 87 filters -
never let the rules stop you experimenting; it's useful to know the
technical limits when playing around though.......




All the best,

             Clive   http://www.cocam.co.uk
                     Photographic Services, Filters and Equipment, Infrared FAQ


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